This invention relates to an improvement in the preparation of an organic isocyanate and, more particularly, to an improved process for preparing an organic isocyanate by the reaction of an organic amine with phosgene wherein the acid substances and hydrolyzable chlorine-containing substances included in the reaction product are minimized.
Organic isocyanates, which are highly reactive compounds, are being widely used for the manufacture of polyurethane foams, elastomers, adhesives, paints, and the like.
While a number of processes for preparing organic isocyanates are known, one of the commonly used processes comprises reacting an appropriate amine with phosgene in the presence of an inert organic solvent to form an isocyanate, degassing the reaction mixture, recovering the solvent, and purifying the reaction product as required. The isocyanate thus obtained, especially a crude one, is contaminated with impurities including acid substances and hydrolyzable chlorine-containing substances. Since such impurities exercise a remarkably adverse effect on the reactivity of the isocyanate in the manufacture of polyurethanes, it is very important to remove or minimize them.
The aforesaid hydrolyzable chlorine-containing substances are considered to result chiefly from various side reactions taking place during the formation of the isocyanate or from the impurities included in the starting materials. For example, in the preparation of polymethylene polyphenyl polyisocyanate, the appropriate polyamine contains trace amounts of secondary amine. This secondary amine reacts with phosgene to form secondary carbamyl chloride.
In addition, various side reactions take place during the reaction of the polyamine with phosgene. For example, the amine reacts with the isocyanate to form polymers having urea(--NHCONH--) residue, which further react with phosgene to form allophanyl chlorides (secondary carbamyl chlorides).
These secondary carbamyl chlorides are relatively stable. That is to say, while the primary carbamyl chloride which is an intermediate in the course of the formation of an isocyanate from an organic amine and phosgene decomposes rather easily at elevated temperatures to give an isocyanate, the secondary carbamyl chlorides are very difficult to decompose.
In order to remove hydrolyzable chlorine-containing substances as described above, it is a common practice to pass nitrogen gas through the reaction product of an organic amine with phosgene at elevated temperatures and thereby drive away the unreacted phosgene, hydrogen chloride, and hydrolyzable chlorine-containing substances dissolved in the reaction product.
The content of such impurities will be substantially reduced if the reaction product treated as above is further distilled prior to practical use. In the case of high boiling isocyanates, however, the reaction product cannot be purified by distillation. Accordingly, it is highly desirable to remove the hydrolyzable chlorine-containing substances without resorting to distillation.
Moreover, in order that such hydrolyzable chlorine-containing substances may be removed as much as possible from the reaction product of an organic amine with phosgene, it is necessary to degas the reaction product at higher temperatures for a relatively long period of time. However, when exposed to high temperatures for a long period of time, isocyanates tend to polymerize and hence cause an undesirable increase in viscosity. Especially in the case of polymethylene polyphenyl polyisocyanate, remarkable changes in viscosity and molecular weight distribution of lower polymer may result.
In order to overcome the above-described difficulties, for example, Japanese Patent Publication No. 29841/'71 discloses a process which involves introducing an inert gas into the reaction product under pressure. Moreover, Japanese Patent Application Disclosure No. 23745/'73 and No. 76839/'73 propose a process which involves passing the reaction product through a packed tower in counterflow with an inert gas stream. However, these processes are undesirable from an economical viewpoint because of an unduly large consumption of inert gas and, moreover, are still incapable of eliminating the disadvantage that the isocyanate tends to polymerize. Thus, they are hardly considered to be practicable in industrial applications.